Sorry to hear about the fleas, Sicy. One other trick, with the vacuum get a flea collar and cut it up into pieces and put it in the vacuum bag. This will kill them while you are vacuuming. (The only decent use of those horrible flea collars!)
With Prego, I wouldn't stop feeding her, just move her food so she isn't hanging out by the door. Hopefully once you gain more of her trust, you can also treat her for worms and fleas on a fairly regular basis and then it will be OK for her to be on your porch.
Early neutering is becoming more and more common, especially for shelter kittens. They have found that too many times if the kitten isn't spayed or neutered when it leaves the shelter, it won't get done. There is also research showing that early spaying and neutering isn't harmful to the cats. Here
is an article on the effects of early spay/neuter.